Anatomy of a pH meter — how many electrochemical cells? A standard laboratory/industrial pH meter uses a measuring (glass) electrode and a reference electrode. Considering their electrochemical arrangement, how many cells are effectively involved?

Introduction / Context:
The pH of a solution is determined electrochemically by measuring the potential difference between a pH-sensitive glass electrode and a stable reference electrode (e.g., Ag/AgCl). Understanding the electrode configuration clarifies calibration, junction potentials, and maintenance requirements of pH probes and meters.

Given Data / Assumptions:

• A typical pH measurement uses a glass (indicator) electrode plus a reference electrode.
• Combination electrodes house both in one body, but internally they remain distinct half-cells.
• The meter reads the EMF between the two, proportional to pH by the Nernst relation.

Concept / Approach:
Each electrode constitutes a half-cell. Together, the glass and reference electrodes form one complete electrochemical cell path through the test solution. In conventional teaching/exam wording, this is often described as “two cells” (two half-cells) used by the pH meter, even when combined in a single probe. Hence, the accepted answer aligns with two cells (two half-cells) forming the measurement system.

Step-by-Step Solution:

Verification / Alternative check:
Manufacturer schematics label “measuring half-cell” and “reference half-cell,” even in combination electrodes. Calibration with two buffer points verifies the cell potential relation to pH.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing “combination electrode” as a single cell; internally it still embodies two half-cells that the meter compares.

Final Answer:
two cells